Optical fibers have become of immense commercial importance in recent years because of their use in optical communication systems. Such communication systems have a number of advantages over conventional communication systems including extremely large bandwidths and high information transmission rates, easy availability of raw material for fiber fabrication, easy adaptability to pulse code modulation transmission, etc. Precise manufacturing standards and structure control are required for the fabrication of optical fibers to ensure transmission of optical signals through such fibers with a minimum of loss and dispersion. Improvements are still being made so as to increase bandwidth, required repeater-station spacing, ease of manufacturing, etc. Indeed, precise design and control of optical fiber properties for various applications are an ongoing activity for those involved in optical fiber manufacture.
At present, there are two types of optical fibers contemplated for use in optical communication systems. One type of fiber, the so-called single-mode fiber, has a relatively small core region and a relatively large cladding region of lower index of refraction. This results in a fiber with characteristics such that only one electromagnetic mode is transmitted with low loss. The advantage of single-mode fibers is their large bandwidth, relatively low dispersion, and relatively large spacing requirements between repeater stations. The other type is the so-called multimode fiber.
In both types of fibers, optimum performance in terms of low loss, low dispersion, high bandwidth, etc., depends critically on the index of refraction profile. This optimum index profile may be different for different wavelengths, types of optical fibers, etc., and may be different for fibers designed for different applications (e.g., maximum bandwidth, maximum distance between repeater stations, etc.).
A variety of methods have been used to make optical fibers. These methods have been reviewed in a number of references including, U.S. Pat. No. 4,257,797 issued to M. J. Andrejco and J. B. MacChesney on Mar. 24, 1981, and U. S. Pat. No. 4,302,230 issued to J. B. MacChesney et al on Nov. 24, 1981.
One particularly advantageous method of fabricating optical fibers is usually referred to as the modified chemical vapor deposition process (MCVD). This process is described in U. S. Pat. No. 4,217,027 issued to J. B. MacChesney and P. B. O'Connor on Aug. 12, 1980, and in U. S. Pat. No. 4,262,035 issued to R. E. Jaeger et al on Apr. 14, 1981. See also U. S. Pat. No. 4,331,462, issued to Fleming et al, May 25, 1982.
This application is only concerned with single mode fiber. As is well known, such fiber generally comprises a central region of relatively high refractive index (the core) that is contactingly surrounded by one or more concentric regions of relatively low refractive index (the cladding). In essentially all communication grade fibers in use today, the core and at least the inner cladding portion consist of low optical loss silica-based material. Such cladding material (herein to be referred to as the "low loss" cladding material) can be produced by any appropriate process, and is currently frequently formed in situ by means of an appropriate reaction and deposited on an appropriate substrate. Currently used fiber typically also comprises an outer cladding portion that consists of relatively high optical loss material (e.g., having a loss in the order of 100 db/km). This "high loss" cladding material can, for instance, be derived from a pre-existing silica-based tube, or from silica-based overclad. Fibers typically are designed such that the "high loss" cladding material plays substantially no part in the guiding of the optical radiation. The radius defining the boundary between the "low loss" and the "high loss" cladding material is herein designated as a.sub.d.
Improvements in the design of single-mode optical fibers are highly desirable both as to improving transmission characteristics (loss, bandwidth, dispersion, etc.), improving the ease of fabricating the optical fiber and reducing the critical nature of the parameters of the optical fiber. In particular, low-loss fiber designs in which the dispersion minimum (or zero) occurs at the wavelength where losses are minimum are highly desirable. Also, for wavelength multiplexing applications, fiber designs which minimize dispersion over a given wavelength range are highly desirable. In addition, it is desirable to adjust the fiber parameters to minimize losses, ensure good mode confinement and to reduce the susceptibility of the fiber to bending loss. Low loss, single-mode fiber structures have been described in a number of references including U.S. Pat. No. 4,435,040 issued to L. G. Cohen et al on Mar. 6, 1984 and U.S. Pat. No. 4,447,127 issued to L. G. Cohen et al on May 8, 1984.
A variety of fiber structures have been described in the literature. Many of these structures have been discussed in a book by L. B. Jeunhomme entitled Single-Mode Fiber Optics, Principles and Applications, Marcel Dekker, Inc., New York, 1983. A particularly interesting single-mode fiber with multipleclad structure is described in U.S. Pat. No. 3,997,241 issued to S. Nishida et al on Dec. 14, 1976. Also of interest is a fiber structure, disclosed by V. A. Bhagavatula in a European patent application published on July 20, 1983 (Application Number 82306476.1), with a core and cladding in which the core includes a depressed refractive index region. See also the European Patent Application Number 84303402.6, published on Dec. 5, 1984.
It is frequently desirable in a single-mode optical fiber used for lightwave communications systems to have a reasonably wide wavelength range with zero or near zero dispersion at a wavelength of low-loss operation (exemplarily including 1.55 .mu.m). It is frequently also desirable with such a design that the optical fiber provide low-loss, single-mode propagation over a reasonably wide wavelength range around the intended operating wavelength of the communications system, have a spot size which permits easy, low-loss coupling between fibers, and exhibit relatively low bending loss.
Among other generally desirably characteristics of a single mode fiber design are relative insensitivity of the propagation characteristics of the fiber to small variations in one or more fiber parameters, and a relatively small slope of the dispersion vs. wavelength curve of the fiber at and near the zero dispersion wavelength .lambda..sub.o of the fiber. Desirably a fiber design also requires only a relatively small thickness of "low loss" cladding, to achieve a predetermined performance specification (typically bending loss and/or attenuation), since the cost of "low loss" cladding material is typically much higher than that of "high loss" cladding material. A still other desirable characteristic of a fiber design is the ability to achieve desired propagation characteristics with a relatively low doping level in the core, since the scattering loss of an optical fiber generally increases with doping level. In some cases it is desirable that a fiber has a relatively broad spectral region of low chromatic dispersion.
In view of the commercial significance of single mode optical fiber, a relatively simple design feature that results in, or improves, one or more of the above described desirable characteristics of optical fiber is of considerable interest. This application discloses fiber designs that comprise such a feature.